Which Issue Prevents Internet Access When Serial IP Is 192.168.7.252/30?
Short answer
D The serial interface was configured with the incorrect subnet mask.
Question
Key Takeaways
The exam tests /30 host boundaries: .252 is the network address, so only .253 and .254 are usable; the trap is assuming any IP can be used with a /30 mask.
In this CompTIA Network+ N10-008 scenario, internet access fails because the serial interface is configured with 192.168.7.252/30. Community consensus selects D: the serial interface's subnet mask is invalid for the IP, turning .252 into a /30 network address rather than a usable host.
Common mistake: Choosing C ('incorrect default gateway') is the most common trap because internet outages feel like routing issues. Actually, a router on this WAN link doesn't use a default gateway to forward packets, and the serial interface's IP/mask combination is clearly invalid.
Expert Analysis
Why the Answer Is Correct
The serial interface is assigned 192.168.7.252 with a /30 mask (255.255.255.252). A /30 subnet contains four addresses: the first is the network address, the next two are usable hosts, and the last is the broadcast address. Since 192.168.7.252 is the network address of the 192.168.7.252/30 subnet, it cannot be assigned to the router's interface. This prevents communication with the service provider, so answer D — the serial interface was configured with an incorrect subnet mask — is the most likely cause. If the administrator intended .252 to be a usable host, they would need a shorter mask such as /29.
Why the Other Options Are Wrong
Option A is unlikely because E1/0's 192.168.8.1/24 is a valid LAN gateway address and the issue is on the WAN side. Option B is irrelevant: loopback addresses are used for router identification or management, not for external internet traffic forwarding. Option C is also less likely because routers do not use a 'default gateway' in the same way hosts do; they use routes, and the broken serial interface makes any default route unusable. The interface output directly points to the invalid serial-interface IP/mask combination.
Community Comment Notes
Several commenters correctly identify the /30 boundary: '252 net addr, 253 first usable, 254 second usable, 255 broadcast' (comment 2). One commenter warns against relying only on ChatGPT because it returned both C and D depending on the prompt, emphasizing the need to understand the subnet math (comment 1). Comment 4 notes that an incorrect serial-interface mask prevents communication with the service provider, while comments 5 and 6 also support D. Although some explanations describe the issue as an 'unusable IP address,' the exam's answer D is accepted because the mask determines that .252 is not a host address.
Official Reference
- https://www.cisco.com/c/en/us/support/docs/ip/routing-information-protocol-rip/13788-3.html
- https://www.comptia.org/certifications/network
These links open the official vendor documentation in a new tab.
Exam Strategy
When you see a /30 serial address, quickly list the four addresses in that block and confirm the router uses one of the two usable hosts. Don't pick 'default gateway' just because internet traffic isn't routing; check the WAN interface's IP address and mask first.
Community Votes
100% of anonymous learners picked answer D. Votes are pick records left by other test-takers — they are not the verified answer.
Discussion 5 contributions
Compiled from public community threads and reader submissions, lightly edited for clarity. Handles and vote tallies are shown as recorded, and comments reflect the views of individual contributors.
From public community threads 5
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stinkle 👍 6Keep a lookout Y'all when using chatGPT. When I asked ChatGPT, it gave me the answer C ! When I asked it a second time with different prompts, it gave the answer as D. Understand WHY it is wrong & not always using the GPT. It's D because the first network address in a /30 is .252, so this is a un-usable address that is assigned to the Serial interface.
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a87d6a4 👍 2 Answered DD. The serial interface was configured with the incorrect subnet mask. An incorrect subnet mask on the serial interface would prevent proper communication between the router and the service provider, affecting internet access. The other options are less likely to be the cause: an incorrect Ethernet IP address would affect local network connectivity, an incorrect loopback address does not impact external network access, and an incorrect default gateway is not relevant for serial link configurations.
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GojoSatori 👍 2D. The serial interface was configured with the incorrect subnet mask. 192.168.8. 192.168.7 = /24 (255.255.255).0 and /30 (255.255.255).252 --- this number inside parenthesis (255.255.255) cant be change so even you configured default gateway .8 to .7 ----- {E1/0: =192.168.8.1 E1/0: 192.168.7.1} it can't be changeable //// you need to correct subnet mask, before you correct the gateway
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Mjsecurity 👍 2 Answered D32-30= 2. 2^2=4 252 net addrs 253 first usable 254 255 broadcast address. the following is the most likely cause is S1: 192.168.7.252/30 -
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Cloud1128 👍 2 Answered Dgpt says correct answer is D
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